FIG. 8 shows a heat insulating board 4 for an exhaust manifold 2 which is used to cut off the heat exhausted from an internal combustion engine. The damaging influence of heat on accessories of the internal combustion engine is decreased by cutting off or reducing the heat emitted from the exhaust manifold 2 by partially enclosing the manifold with the heat insulating board 4.
The above heat insulating board 4 for the exhaust manifold 2 is disclosed in Japanese Publication Nos. 60-122524 and 61-65231.
The heat insulating board disclosed in the earlier publication is utilized by using bolts to secure the heat insulating board to the exhaust manifold and press-fitting the exhaust manifold to the heat insulating board when fastening the bolts.
The heat insulating board disclosed in the latter publication is made by providing a space between the inside and outside materials of the heat insulating board and installing many heat-releasing projections on the outside material.
The known heat insulating board 4 for the exhaust manifold 2 as shown in FIG. 8 includes, for example, the type shown in FIGS. 5 through 7.
The heat insulating board 4 consists of an inside material layer 8 and outside material layer 10 which are laminated with a heat insulating material layer 6 as shown in FIG. 6. The board 4 is, as shown in FIG. 5, made by fixing together the peripheral area 12 of the inside material layer 8 and the outside material layer 10 by using the caulked connecting tab portions 14. That is, the tabs 14 on the inside layer 8 project over the edge portion of the outside layer 10 and are fixed thereto, as by being caulked thereto. The board 4 is also provided with a depression 18 at the internal area 16, excluding the peripheral area 12, of the inside material 8 and the outside material 10.
However, in this conventional heat insulating board 4, connecting tab portions 14 are not located directly adjacent or aligned with the adjacent end wall 18e of the depression 18, but rather are disposed outwardly along the periphery 12 on opposite sides of this end wall 18e. The portion 20 directly adjacent the end wall 18e is not fixedly coupled to the other layer. Therefore, thermal stresses, due to thermal deformation of the above depression 18, are concentrated on the peripheral area 12 facing or adjacent the end 18e, and cause cracks in the interface 22 between the connecting portion 14 and the non-connecting portion 20, thereby shortening the life of the heat insulating board 4.
Although reinforcement can be used to prevent the above-described cracks, the cost of manufacturing the board 4 will increase.